Mrs Obasanjo, 59, was rushed to a hospital emergency department in the southern Spanish resort of Marbella early on Sunday after an operation, reportedly to reduce weight, at the town's Molding Clinic. The Nigerian first lady was dead on arrival. Attempts to resuscitate her failed.

She was reported to have undergone an operation on Saturday, though it was unclear exactly what the first lady had wanted to change. Nigerian newspapers said she had had a "tummy tuck".

Forensic scientists said they had been ordered to investigate whether her death had been caused by medical malpractice. The Molding Clinic is one of several centres to have opened in Marbella in recent years, offering "health tourism" visits to the glitzy Costa del Sol resort which is more usually associated with minor film stars and international crooks. Yesterday the clinic released a statement saying the first lady "did not die while being operated on". It said the "fundamental causes" of her death had not yet been determined.

However, the USP Hospital, where Mrs Obasanjo was sent after her recovery process began to go wrong, said she was "clinically dead" when she arrived.

Nigerian newspapers yesterday reported that the woman they called "Chief" Obasanjo had appeared to be in good health before she travelled to Spain. When she last appeared in public a week ago she had danced at a wedding, Nigeria's Tribune newspaper reported.

President Obasanjo received news of her death as he was dealing with a national tragedy, following the crash of a Boeing 737 airliner that killed all 117 people aboard. Nigerian authorities did not admit that the country's first lady had died after visiting a plastic surgeon.

A presidential spokeswoman, Oluiremi Oyo, went on television to tell Nigerians that the president's "beloved wife died in Spain early this morning after surgery".

Mrs Obasanjo became known around the world after she campaigned for the release of her husband, a former army general, when he was jailed in the mid-1990s for allegedly plotting a coup. She received a number of human rights awards.

The luxurious-looking Molding clinic is housed in a vast neo-classical building fronted by towering palm trees in one of the more expensive parts of a resort that is a well-known favourite with Arab potentates and developing world dignitaries. Among other products, it advertises a facial transformation that it calls "The Molding Mask". The Molding Clinic said yesterday it was co-operating fully with Spanish authorities while awaiting the postmortem.

Spain is said to have the highest number of plastic surgeons per head in Europe, with Spaniards most likely to queue up for treatment by scalpel, laser or liposuction machine. Demand for cosmetic surgery has boomed despite the recent deaths of several patients who have undergone liposuction or stomach-stapling. The country's biggest cosmetic surgery company, Corporación Dermoestética, recently floated shares and has already expanded into Britain.

An autopsy on Mrs Obasanjo was completed yesterday afternoon and a report sent to the local court, Antonio García de Gálvez, director of the Malaga Institute of Forensic Medicine, told journalists. He confirmed that the court was investigating the possibility of medical malpractice but would not say what kind of cosmetic procedure Mrs Obasanjo had undergone.

Plastic people

· Britons spend more than £200m a year on cosmetic surgery, while the industry pays £1m a year in compensation to dissatisfied customers

· Nearly half of all British women and a quarter of men would consider plastic surgery, says a YouGov poll

· The cost of surgical treatment in the UK ranges from £3,500 to £12,000

· More than 2m cosmetic surgery operations were carried out in the US last year

· The most popular US operations were liposuction, breast implants, eyelid surgery, nose jobs, and facelifts